This invention relates to an apparatus for the production of molten metals from their ores, and more specifically to a metal-making apparatus of this general nature based upon the technology of smelting reduction of ores containing oxides of the desired metals. Still more specifically, the invention pertains to such a metal-making apparatus making possible the use of ores in a wide range of particle size.
The smelting reduction method is one whereby metals such as iron and ferroalloys are produced by reductive treatment of iron oxide ores or the like in a molten state. The metal-making industry has devoted research and development efforts to the enhancement of the practical utility of this method because of its potential capability of meeting the foreseeable future trend of materials and energy supply.
Among the advantages of the smelting reduction method are, as far as iron making is concerned: (1) cheapness of the raw materials as compared with those required for the blast furnace method; (2) elimination of pretreatments such as sintering or pelletizing of too fine ore particles; and (3) compactness of the equipment required. Additionally, for the production of ferroalloys, the metal promises greater independence from electric energy than most other known methods.
While a variety of suggestions and proposals have so far been made for the practice of the smelting reduction method, the current trend in the industry is toward the use of two furnaces in tandem, one for preliminary reduction of the ore in a solid state and the other for smelting reduction. There are many variations in this tandem-furnace method, involving different furnace types and different heat production methods.
Generally, such known tandem-furnace methods are preferred by reason of the regenerative process involved; that is, the prereduction of the solid-state ore is possible by making use of the heat and reductive capability possessed by the waste gas generated from the smelting reduction furnace. The reductive gas is generated at high temperatures as a result of the reduction taking place within the smelting reduction furance as coil or like material and oxygen gas are introduced into the metal bath therein. Upon withdrawal from the smelting reduction furnace, the reductive high-temperature gas is directed into the prereduction furnace for the preliminary reduction of the ore in the solid state.
The general belief of specialists is that the prereduction furnace should advantageously be of the so-called "fluidized bed" or fluosolids type, provided that the ore is more or less in a state of fine particles. This type of furnace gives the properties of a quasifluid to the ore particles introduced therein, making possible, the continuous processing of the pulverized material. As additional advantages, the complete charge of ore particles within the furnace can be maintained at a constant temperature, and the ore particles make intimate contact with the reducing gas.
A typical prior art prereduction furnace of the fluidized bed type is found in Japanese Laid Open Patent Application No. 58-217615. It comprises a vessel in the form of an upstanding cylinder, with an ore supply chute and a gas exhaust conduit coupled to its top portion, and with a reducing gas supply conduit and an ore discharge chute coupled to its bottom portion. The furnace vessel has a gas distributor in the form of a grate or perforated bottom laid horizontally above the intake port of the reducing gas.
Ore in the state of fine particles is charged onto the gas distributor of the furnace, whereas the high temperature reducing gas is introduced into the furnace through an inlet port positioned under the gas distributor. Stirred by the reducing gas blasted up through the gas distributor, the ore particles will become "fluid" enough to make intimate contact with the gas thereby undergoing the desired process of prereduction. The material in this condition forms a "fluidized bed", with the reducing gas "bubbling up" therethrough. After having been thus prereduced, the ore particles will be discharged from the prereduction furnace and recharged into the smelting reduction furnace for final reduction in the molten state.
As so far constructed, however, the fluidized-bed type prereduction furnace has had a weakness. It imposes strict limitations upon the particle size of the ore in order to form a sufficiently "fluid" mass of the ore particles within the furnace. The reducing gas must be introduced at a rate depending upon the particle size of the ore in order to impart sufficient "fluidity" to the ore particles. Therefore, the prior art prereduction furnace does not lend itself to the processing of ore particles which differ in size over a wide range. The particle size has had to be not more than three millimeters for successful prereduction. Moreover, depending upon the particular prereduction process employed and the particular kind of ore to be processed, additional limitations have been imposed on the average particle size and on the percentage of very fine particles present.
The inconveniences arising from such limitations of the prior art fluidized-bed type prereduction furnace will become apparent in the light of the fact that the iron ores available commercially as the raw materials of iron manufacture contain a considerable percentage (e.g. 30 percent) of particles exceeding three millimeters in size. Even particles of 10 millimeters or more in size are present.
A conventional solution to this problem has been the sizing of ore particles by screening. Particles too coarse to be treated by the prereduction furnace of the fluidized-bed type have had to be recomminuted into the required size. Alternatively, if such coarse particles are not to be recomminuted, some other reduction means such as a shaft furnace have had to be employed. In any event, for making full use of the materials purchased, the prior art fluidized bed furnace has required some additional means such as screens and crushers, or another reducing furnace, thus incurring additional installation and running costs, and additional manufacturing steps.
There have also been some problems left unsolved in conjunction with the smelting reduction furnace to be connected in tandem with the solid-state prereduction furnace. The following two methods of charging ore into the furnace have been suggested:
1. The gravity charging of the whole ore particles from the top portion of the furnace.
2. The carrier-gas charging of the whole ore particles through a nozzle coupled to either the midportion or bottom portion of the furnace (Japanese Laid-Open Patent Application No. 59-113110).
The first described method of gravity charging is objectionable because the finer ones of the ore particles, on being introduced into the smelting reduction furnace, tend to be blown out of the furnace by the gas generated therein. It may be contemplated to avoid such waste of the material by previously screening out the finer particles or by pelletizing the fine particles into larger ones by use of a binder. But then these measures demand the provision of screens or like sizing means, or means for pelletizing, as well as additional processing steps and additional processing time.
The second recited method of carrier-gas charging, although free of the noted drawbacks of the gravity charging method, has its own shortcomings. If the pulverized material contains too coarse particles or lumps, they may clog up the charging nozzle or the conduit leading to the nozzles. It would be no satisfactory solution to make the nozzle and conduit large enough to permit the passage of such coarse lumps. For such large nozzle and conduit would require a corresponding increase in the flow rate of the carrier gas, possibly resulting in a bad effect to the reduction reaction and an undue drop in the bath temperature or in the blowing of the carrier gas through the metal bath. The usual practice, therefore, has been to pulverize the ore into sufficiently fine particles to preclude the possibility of clogging. This practice is also objectionable because of such additional means required as crushers and screens, and of the additional operation necessitated.